Double metal cyanide (DMC) catalysts for polyaddition reactions between alkylene oxides and starter compounds having active hydrogen atoms are known (for example, U.S. Pat. No. 3,404,109, U.S. Pat. No. 3,829,505, U.S. Pat. No. 3,941,849 and U.S. Pat. No. 5,158,922). The utilization of these DMC catalysts for the preparation of polyether polyols in particular brings about a reduction in the proportion of monofunctional polyethers having terminal double bonds, so-called monools, by comparison with the conventional preparation of polyether polyols by means of alkali catalysts, such as alkali metal hydroxides. The polyether polyols thus obtained can be processed to form high-grade polyurethanes (for example elastomers, foams, coatings). DMC catalysts are normally obtained by reacting an aqueous solution of a metal salt with the aqueous solution of a metal cyanide salt in the presence of an organic complexing ligand, for example an ether. In a typical catalyst preparation, for example, aqueous solutions of zinc chloride (in excess) and potassium hexacyanocobaltate are mixed, and dimethoxyethane (glyme) is then added to the suspension formed. Following filtration and washing of the catalyst with aqueous glyme solution an active catalyst corresponding to the general formulaZn3[Co(CN)6]2 xZnCl2 yH2Ozglymeis obtained (see, for example, EP-A 700 949).
JP-A 4 145 123, U.S. Pat. No. 5,470,813, EP-A 700 949, EP-A 743 093, EP-A 761 708 and WO 97/40086 disclose DMC catalysts which further reduce the proportion of monofunctional polyethers having terminal double bonds when polyether polyols are prepared, by the utilization of tert.-butanol as an organic complexing ligand (alone or in combination with a polyether (EP-A 700 949, EP-A 761 708, WO 97/40086)). Moreover, the utilization of these DMC catalysts reduces the induction time in the polyaddition reaction of the alkylene oxides with corresponding starter compounds and increases catalytic activity.